This invention relates to a novel photosensitive resistive ink composition, which can be used for forming a permanent protective mask, such as solder resist or the like, to be employed for the production of printed circuit boards (printed wiring boards), has high sensitivity and high resolution (line width 50 xcexcm) and high speed curability to a ultraviolet irradiation, and is developable with a dilute alkaline aqueous solution and forms by curing a dry film excellent in electroless gold plating resistance, soldering heat resistance, electrical characteristics and mechanical characteristics. In addition, it""s easy to inspect the appearance by a formed photoresist film having a bright and transparent property.
Due to the recent trend toward smaller size and higher functionality of electronic equipment and apparatus and toward higher connection density, higher pin and refinement of electronic packing technology, solder mask has been required to be of higher resolution, accuracy and reliability to match the refined surface mount technology. Various improvements in the solder mask have been proposed. Solder mask has been improved from a screen printing method having low resolution and high pollution disadvantages to a liquid photo-imaginable solder mask method developable with a dilute alkaline aqueous solution and having high resolution and low pollution advantages. However, there are many drawbacks in the liquid photo-imaginable solder mask. For instance, U.S. Pat. No. 4,943,516 proposes a resin composition comprising an undissolved epoxy compound to improve photosensitivity and decrease to react with a curing agent. A photo-curable prepolymer has a higher acid value to dissolve in a dilute alkaline aqueous solution, but it causes the poor resistance to plating. Moreover, because of the network structure after photoreaction, the undissolved epoxy compound reduces the opportunity of reacting with a curing agent to cause the poor resistance to heat.
The major object of this invention is to overcome the drawbacks mentioned above and to provide a novel photosensitive resist ink composition useful as a liquid photo-imaginable solder mask, which has superior ultraviolet curability, soldering eat and chemical resistance, electroless gold plating resistance, flexibility, electrical characteristics and is developable with a dilute alkaline aqueous solution and also has bright and transparent appearance.
This invention relates to a liquid photosensitive resistive ink composition developed with a dilute alkaline aqueous solution, which comprises:
(A) a photo-curable prepolymer represented by the formula I, obtained by the reaction of (a) epoxy resin containing at least two terminated epoxy groups and (b) a,xcex2-unsaturated monocarboxylic acid containing at least one vinyl group, and then reacting (c) a saturated or unsaturated polybasic acid anhydride.
(B) a photo-curable monomer containing at least three vinyl groups and an a,xcex2-unsaturated carboxylic acid represented by the formula II,
(C) a photo-initiator,
(D) organic solvents,
(E) an epoxy compound containing at least one vinyl group and one epoxy group in the molecular unit represented by the formula III,
(F) a curing agent capable of allowing a prepolymer (A) undergo a thermal reaction, and
(G) a clay nano-composite.
One characteristic of the invention is in the prepolymer (A) comprises at least one vinyl group in a molecular unit. The vinyl group increases the photocuring strength, resolution and resistance to plating of a liquid photo-imageable solder mask. In addition, the photo-curable monomer (B) of the invention comprising hydrophilic carboxylic acid group increase the solubility difference between the exposed area and unexposed area. And it can increase the developability and resolution. Moreover, the epoxy compound (E) containing at least one vinyl group and one epoxy group in the molecular unit offers steric hindrance which prevents the reaction (E) with a curing agent and a photo-curable pre-polymer. The vinyl group of the compound (E) can be reacted with the photocurable prepolymer to increase the opportunity of the thermal reaction during curing and to enhance heat resistance. The compound (E) can be dissolved in organic solvents and a dilute alkaline aqueous solution to make the appearance bright and crystalline and to be easy to develop.
The liquid photo-curable pre-polymer (A) of the invention by the formula (I) as follow, which is a reaction product obtained by reacting (a) an epoxy resin containing at least two terminated epoxy groups and (b) an unsaturated monocarboxylic acid containing at least one vinyl group, and then reacting with a saturated or unsaturated carboxylic acid anhydride. 
m,n: repeating units, represented by integers as m+nxe2x89xa620
R1: xe2x80x94H, xe2x80x94CH3 
xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94
(a) of at least two epoxy compounds which can be used in this invention are represented as phenol novolac epoxy resins, cresol novolac epoxy resins, halogenated phenol novolac epoxy resins. Among these epoxy resins, phenol novolac epoxy resins and cresol novolac epoxy resins are preferred.
(b) xcex1,xcex2-unsaturated monocarboxylic acid containing at least one vinyl linkage can be included as: acrylic acid, methacrylic acid, crotonic acid and cinnamic acid. Among these unsaturated monocarboxylic acid mentioned above, acrylic acid is particularly preferable.
The saturated or unsaturated anhydride (c) can be selected from succinic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, tetrahydrophthalic anhydride and maleic anhydride. Among these saturated or unsaturated anhydride mentioned above, tetrahydrophthalic anhydride is particularly preferable.
According to above mentioned discussion, the amount and ratio of each component could get the best result in the following region. In the reaction, the suitable proportion of an unsaturated monocarboxylic acid containing at least one vinyl group taking part in the reaction is 0.5 to 1.2 mole, preferably 0.9 to 1.1 mole, to 1 equivalent of the epoxy groups of the epoxy compound. The proportion of polybasic acid anhydride (c) is 0.1 to 1.0 mole, preferably 0.3 to 0.5 mole, to 1 equivalent of the hydroxyl group produced by reacting above-mentioned acid with the epoxy compound.
The liquid photo-curable prepolymer (A) of this invention is obtained by the reaction of an epoxy resin containing at least two terminated epoxy groups and an xcex1,xcex2-unsaturated monocarboxylic acid containing at least one vinyl group. In the reaction, the suitable examples of a catalyst which can be used in this invention include triphenylphosphine, triethylamine, methyl triethyl ammonium chloride and so on, preferably triphenyl phosphine. The catalyst may be used preferably in the amount of 0.1 to 10 parts by weight. In order to prevent polymerization, add thermal polymerization inhibitors such as hydroquinone, hydroquinone monomethyl ether preferably. The thermal polymerization inhibitor may be added preferably in the amount of 0.01 to 1 part by weight, per 100 parts by weight of the reaction mixture. The suitable reaction temperature and reaction time may be controlled between 80 and 120xc2x0 C., 6 and 24 hours, separately. In addition, in the above mentioned reaction product reacting with a saturated or unsaturated polybasic acid anhydride, the suitable reaction temperature and reaction time may be controlled between 80 and 130xc2x0 C., 4 and 16 hours, separately.
The acid value of the photo-curable prepolymer (A) so obtained suitably falls in the range of 20 to 120 mg KOH/g, preferably 30 to 80 mg KOH/g.
The photo-polymerizable prepolymer (A) in the invention can be used in an amount of 10 to 80 parts by weight, preferably 30 to 50 parts by weight of liquid photosensitive resistive ink composition.
The photo-curable monomer (B) containing at least three vinyl groups and an xcex1,xcex2-unsaturated carboxylic acid is prepared by reacting dipentaerythritol pentaacrylate or pentaerythritol triacrylate with polybasic acid anhydride. Typical representative acid anhydride, which can be used in this invention, includes succinic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, tetrahydro-phthalic anhydride and maleic anhydride. Preferably is the product of pentaerythritol triacrylate reacting with tetrahydrophthalic anhydride.
The aforementioned unsaturated monocarboxylic acid (B) cannot be available from commercial market. It can be synthesized by reacting dipentaerythritol penta-acrylate or pentaerythritol triacrylate with polybasic acid anhydride. The proportion of the polybasic acid anhydride taking part in the reactions is 0.5 to 1.1 equivalents, preferably 0.7 to 0.9 equivalent, to 1 equivalent of hydroxyl groups of pentaerythritol acrylate. The suitable examples of a catalyst, which can be used in this invention, include triphenylphosphine, triethyl amine, methyl triethyl ammonium chloride and so on, preferably triphenylphosphine (TPP). The catalyst may be used preferably in an amount of 0.1 to 10 parts by weight of liquid photo-polymerizable resinous composition. In order to prevent polymerization, add thermal polymerization inhibitor such as hydroquinone, hydroquinone monomethyl ether preferably. The thermal polymerization inhibitor may be added preferably in the amount of 0.01 to 1 part by weight of the reaction mixture. The suitable reaction temperature and reaction time may be controlled between 80 to 120xc2x0 C., 20 to 48 hours, separately. The reaction product is represented by the formula II, as follow: 
The above mentioned unsaturated monocarboxylic acid (B) in the invention can be used in an amount of 1 to 40 parts by weight, preferably 5 to 20 parts by weight of liquid photosensitive resistive ink composition.
Photo-polymerizable initiator includes any known photo-polymerizable initiator, such as: benzoin methyl ether, benzoin isopropyl ether, 2,2-dimethoxy-2-phenyl-acetophenone, 1,1-dichloro-acetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpho-lino-propane-1-one, N,N-dimethyl amino-acetoohenine. 2,4-dimethylthioxanthone, 2,4-diethvthioxanthone 2-chloro-thioxanthone, 2,4-diisopropylthioxanthone, acetophenone dimethyl ketal, benzyl dimethyl ketal, methyl benzophenone, 4,4xe2x80x2-dichloro-benzophenone, 4,4xe2x80x2-bis-diethyl-aminobenzophenone, Michler""s ketone and the like. These compounds may be used alone or as a mixture of two or more to dissolve the above-mentioned composition. It is most preferred to employ the combination of 2-methyl-1-[4-(methylthio)phenyl]-2-morpho-lino-propane-1-one, (Irgacure 907, manufactured by Ciba-Giegy Co. Ltd.), and 2,4-diisopropylthioxanthone (Kayacure ITX, manufactured by Nippon Kayaku Co. Ltd.)
The photo-polymerizable initiator (c) can be used in an amount of 0.5 to 10 parts by weight, preferably 1 to 5 parts by weight of liquid photo-polymerizable resinous composition.
As for the organic solvent (D) used as diluent in this invention, the suitable examples of organic solvents include propylene carbonate, butyl cellosolve, butyl cellosolve acetate, toluene, xylene, butyl carbitol acetate, cyclohexanone, propylene glycol monomethyl ether, dipropylene glycol diethyl ether, methyl carbitol acetate and so on. Preferable examples can be propylene carbonate and butyl cellosolve acetate.
The organic solvent (D) in this invention can be used in an amount of 15 to 60 parts by weight, preferably 25 to 50 parts by weight of liquid photo-polymerizable resinous composition.
The another characteristic of the invention is the epoxy compound (E) containing at least one vinyl group and an epoxy group in the molecular unit. The epoxy compound which can be dissolved in the organic solvents is a transparent liquid at room temperature and has increased the photosensitivity due to ethylenic reactions possible. Due to small molecular weight, it is easy to be developed with a dilute alkaline aqueous solution. In addition, it is difficult to react with a curing agent or a photo-curable pre-polymer owing to steric hindrance caused by its structure. The compound (E) containing a vinyl group can be also reacted completely with the photo-curable prepolymer during the thermo-curing to cause a good heat resistance and excellent transparency.
The desirable examples of the epoxy compound satisfying all these requirements can be included as bis-phenol S type resins, heterocyclic epoxy resins, bixylenol type epoxy resins, bis-phenol type epoxy resins and tetraglycidyl xylenoyl ethane resins. Among these epoxy resins mentioned above, bixylenol type epoxy resins are particularly desirable.
An xcex1,xcex2-unsaturated monocarboxylic-acid containing at least one vinyl group can be included as acrylic acid, methacrylic acid, crotonic acid and cinnamic acid. Among these unsaturated monocarboxylic acid mentioned above, acrylic acid proves particularly suitable. The proportion of the monocarboxylic acid taking part in the reactions is 0.05 to 1.1 equivalents, preferably 0.1 to 0.5 equivalent, to 1 equivalent of the epoxy group of the epoxy resins. The suitable examples of catalyst in this invention include triphenylphosphine, triethylamine, methyl triethyl ammonium chloride and so on, preferably triphenylphosphine. The catalyst may be used preferably in an amount of 0.1 to 10 parts by weight, per 100 parts by weight of liquid photopolymerizable resinous composition. In order to prevent radicals caused by double bond in polymerization, add thermal polymerization inhibitors such as hydroquinone, hydroquinone monomethyl ether preferably. The thermal polymerization inhibitor may be added preferably in the amount of 0.01 to 1 part by weight, per 100 parts by weight of the reaction mixture. The suitable reaction temperature and reaction time may be controlled between 80 and 120xc2x0 C., 6 and 12 hours, separately. The reaction product is represented by the general formula III, as follow: 
The aforementioned epoxy compound (E) containing at least one vinyl group and one epoxy group in the molecular unit in this invention can be used in an amount of 1 to 60 parts by weight, preferably 5 to 20 parts by weight, per 100 parts by weight of liquid photopolymerizable resinous composition.
As for a curing agent (F) capable of allowing a prepolymer (A) undergo a thermal reaction, it can include any known epoxy compound containing at least two epoxy groups. The suitable examples are triglycidyl isocyanate, phenol novolac epoxy resins, cresol novolac epoxy resins, halogenated phenol novolac epoxy resins, bis-phenol A epoxy resins, bis-phenol F epoxy resins, bis-phenol S epoxy resins, triphenolmethyl epoxy resin, tetrabromo bis-phenol A epoxy resins and so on.
The above mentioned curing agent (F) capable of allowing a prepolymer (A) undergo a thermal reaction can be used in an amount of 0.1 to 15 parts by weight, preferably 0.5 to 5 parts by weight, per 100 parts by weight of liquid photosensitive resistive ink composition.
The major filler used in this invention includes a well-known inorganic filler such as barium sulfate, talc, magnesium carbonate, calcium carbonate, aluminum oxide or silicon dioxide. The filler in this invention can be used in amount of 20 to 40 parts by weight, preferably 25 to 35 parts by weight, per 100 parts by weight of the liquid photopolymerizable resinous composition. In addition, the clay nano-composite (G) intercalated by the quaternary ammonium salt can be added or reacted with an epoxy resin and a curing agent to obtain clay of nano-meter (10xe2x88x929 m) level. The quaternary ammonium salt is represented by CH3(CH3)nNR3+and the suitable number of n is 3 to 17, preferably 11 to 17 and R represents hydrogen or methyl.
The suitable epoxy resins include phenol novolac type epoxy resins, cresol novolac epoxy resins, halogenated phenol novolac epoxy resins, bis-phenol A epoxy resins, bis-phenol F epoxy resins, bis-phenol S epoxy resins, triphenolmethyl epoxy resins, tetrabromo bis-phenol A epoxy resins and so on. Among these epoxy resins, bis-phenol A epoxy resins and bis-phenol F epoxy resins are particularly preferable. The curing agent can be used in this invention includes primary, secondary and tertiary amine, polythiokol, polyacid, acid anhydride and so on. Particularly preferable is primary amine. Because the clay nano-composite (G) can be dispersed in the composition with a nano-meter particle after curing, an amount of inorganic filler not only can be decreased, but also good heat resistance, scrape resistance, moisture resistance, impact strenght of this composition can be obtained while the clay nano-composite (G) used in this way has no influence on the transparency and gloss of the composition. The clay nano-composite in this invention can be used in an amount of 0.001 to 12.5 parts by weight, preferably 0.05 to 2% by weight of the liquid photosensitive resistive ink composition.
In addition, well known and widely used additives such as coloring pigment represented by phthalocyanine blue, phthalocyanine green, iodine green, diazo yellow, crystal violet, titanium dioxide, carbon black and naphthalene black, thermo-polymerization inhibitors represented by hydroquinone and hydroquinone mono-methyl ether, anti-foaming agents represented by silicone type and fluorine type, and leveling agents, and thickening agents represented by clay and fumed silica. Moreover, add melamine in a suitable amount of 0 to 3% by weight of the liquid photopolymerizable resinous composition to increase the cross-linking density of the composition.
Preferred Embodiments
Now the invention, a photosensitive resistive ink composition comprising (A) a photo-curable prepolymer, (B) a photo-curable monomer containing at least three vinyl groups and an xcex1,xcex2-unsaturated carboxylic acid, and (E) an expoxy compound containing at least vinyl group and an epoxy group, will be described more specifically below with reference to preparation examples. It should be noted, however, that the present invention is not limited by these examples. Whenever xe2x80x9cpartsxe2x80x9d and xe2x80x9c%xe2x80x9d are mentioned, these are invariably used on weights unless otherwise specified.